1. Field of the Invention
The present invention relates to an internal combustion engine equipped with an electronically controlled throttle system opened and closed by an actuator in order to accomplish a target position of the throttle valve disposed in an intake system and, particularly, to fail-safe control technology at a time when the sensors constituting the system become abnormal.
2. Related Art of the Invention
There has been proposed an electronically-controlled throttle system for electronically controlling the position of the throttle valve to obtain a target air quantity based on the position of the accelerator (depressed amount of the accelerator pedal) or on the position of the accelerator and the engine rotation speed (see Japanese Unexamined Patent Publication No. 7-180570).
Among such electronically controlled throttle systems, in case the drive system fails to operate, those (fully electronically-controlled throttle systems) without a limp-home mechanism for mechanically linking the throttle valve by the accelerator operation through a wire employ the below-mentioned system.
That is, provision is made of two accelerator position sensors and two throttle position sensors. As for the position of the accelerator, the smaller value is selected between the two detection values (to prevent the output from becoming excessive). As for the position of the throttle valve, the detection value of the main throttle position is used and, depending upon the cases, the larger value is selected between the two detection values (selecting the larger value effects the correction toward the decreasing side due to the feedback control, and prevents the excessive output).
In case one of the two accelerator position sensors or one of the two throttle position sensors fails to operate, the output from the actuator is stopped, and the throttle valve is linked between two springs (return spring and default spring) and is held at a predetermined default position at which these springs are balanced, in order to maintain a so-called limp-home state (low-speed fail-safe operation of a minimum compensation capable of traveling with the minimum output). In case the one sensor fails to operate, if the throttle valve position is controlled using the remaining sensor, there may take place acceleration or deceleration due to the unintended opening/closing operation of the throttle valve in case the remaining sensor also fails to operate.
According to the fail-safe system in the above-mentioned fully electronically-controlled throttle system, however, if the one sensor fails to operate, the throttle valve is forcibly maintained the fail-safe position at least at that moment without utilizing the value detected by the remaining normal sensor, causing such an inconvenience that the travelling can only be performed at, for example, 40 kilometers/hour at the fastest.
Besides, among the parts constituting the electronically-controlled throttle system, the accelerator position sensor and the throttle position sensor are likely to fail to operate. It has, therefore, been demanded to guarantee traveling performance of some degree at a time of single-failure of these sensors.
The present invention was accomplished by giving attention to the above-mentioned problem inherent in the prior art, and has an object of controlling the operation at a desired speed (fail-safe control in case of single-failure=limp-home control) by using a value detected by the remaining sensor (at a time of single-failure of the sensor) if the remaining sensor is normal, while maintaining a low-speed fail-safe operation of the minimum compensation.
Another object of the invention is to smoothly take over the single-failure fail-safe control operation when the single-failure occurs in the sensor.